Applicant has previously developed temperature control units utilizing pressurized liquid refrigerant, expansion valve devices, and heat exchangers/evaporators to provide the thermal capacity needed for cooling or heating thermal transfer fluid that flows within a process tool, in order to maintain the tool at a selected temperature level. The units function with high thermal efficiency, provide precise control, and rneet the demanding needs of modern high-capital intensive industries, such as semiconductor industries using cluster tools. For such applications, long life and high reliability are essential, but the requirements also include compactness and small footprint because of the high costs of floor footage in such facilities.
These industries are continually evolving and developing more demanding applications which need more versatile temperature controls but at the same time at lower cost. More particularly, such installations now demand selectable refrigeration and optional heating of thermal transfer fluid in the range from about −80° C. to about +60° C., with precision and efficiency. It should be intuitively evident that such a wide temperature range cannot be met economically by conventional refrigeration systems. One approach to the problem of operating over a range of refrigeration temperatures is that proposed by Mizuno et al in U.S. Pat. No. 4,729,424 wherein a cascaded series of refrigeration units are employed. Each unit supplies its own refrigeration capacity as commanded by a central system, to provide stepwise refrigeration capability. Temperature levels between the different refrigeration increments are established by heating within the incremental range. The use of a number of refrigeration units (four in the Mizuno et al proposal) presents particular problems in terms of space requirements, efficiency and reliability. Also, refrigeration units, for long life, should not be run intermittently. Any specific refrigerant further imposes some inherent limitation, depending upon its critical temperature, on the range of operation. In addition efficiency is inherently reduced when heating must be employed to counteract over-cooling.